An ultrasonic device for detecting a direction to an object is proposed. The device transmits an ultrasonic wave pulse as a transmitting wave. The transmitting wave is reflected on the object. The reflected wave is received by multiple receiving elements, which are arranged in an array. A phase difference between a phase of the reflected wave received by one of the receiving elements and a phase of the reflected wave received by another one of the receiving elements, a distance between the one and the other one of the receiving elements, and a wavelength of the transmitting wave are used for detecting the direction of the object.
When the ultrasonic wave is used, it is necessary to expose the transmitting element and the receiving element to an outside of the device. When the direction of the object is detected based on the phase difference (i.e., the phase difference between two receiving elements), a phase of each element shifts if temperature characteristics of a receiving side of the device may be varied or a foreign particle such as a water drop or a dust is attached to the receiving element. Thus, detection accuracy of the device is reduced because of the phase shift.
JP-A-2006-242650 corresponding to US 2006/0196272 teaches that operation failure is detected by monitoring the reflected wave received by the receiving elements. JP-A-2007-93480 teaches that an electric wave is used as the transmitting wave, the reflected wave from a radome arranged in front of the device is received, a phase difference caused by variation of temperature characteristics of the receiving side of the device is stored as an initial phase difference, and the actual phase difference is calculated based on the detected phase difference and the initial phase difference.
However, when the receiving elements are exposed to the outside of the device, the technique disclosed in the above prior art does not completely settle the matter of reduction of the detection accuracy for detecting the direction of the object, the reduction being caused by the variation of the temperature characteristics of the receiving side of the device or the foreign particle attached to the receiving elements.